G.711 is the international standard for encoding telephone audio on 64 kbps channel. G.711 is a pulse code modulation (PCM) scheme operating at an 8 kHz sample rate, with 8 bits per sample. G.711 is an ITU-T standard for audio companding, and is primarily used in telephony. According to the Nyquist theorem, which states that a signal must be sampled at twice its highest frequency component, G.711 can encode frequencies between 0 and 4 kHz. G.711 is a standard to represent 8 bit compressed pulse code modulation (PCM) samples for signals of voice frequencies, sampled at the rate of 8000 samples/second and 8 bits per sample. A G.711 encoder will thus create a 64 kbit/s bitstream.
G.722 is an ITU-T standard wideband speech codec operating at 32-64 kbit/s. G.722 offers lower bit-rate compressions than G.711. The unencoded signal is 14 bits at 16 KHz, producing an effective bandwidth of 50 Hz to 7000 Hz (per the G.722 standard).
There are two different companding variants of G.711: μ-law (a.k.a. “Mu-Law”) and A-law. The μ-law algorithm is used in North America & Japan and A-law algorithm is used in Europe and the rest of the world. Both are logarithmic, but the A-law was specifically designed to be simpler for a computer to process. The standard also defines a sequence of repeating code values which defines the power level of 0 dB.
In digital signal processing, a Quadrature Mirror Filter (QMF) is used to split an input signal into bands (e.g., a high band and a low band) which are then usually sub-sampled by a factor of two. The QMF includes a high-pass filter and a low-pass filter wherein the sum of the magnitudes response of the filters is equal to one at every frequency.